Entering tags, old and new tags, has been quite a battle for a long time. Here's what I've observed:
Fairly frequently, but not always, when I type part of a tag into one of the tag input fields and stop part way through, what I've typed disappears. The little dropdown of matched/recommended tags usually remains.
Sometimes my typed tag text disappears within a fraction of a second, leaving behind the dropdown, sometimes after a couple of seconds, and sometimes not at all.
Sometimes Evernote's disappearing of typed tag text is so aggressive that I have to reenter the letters 5